In order to communicate over a wireless or wired channel, it is common practice to divide the frequency spectrum into multiple channels (frequency division multiplexing). Data is modulated into a specific channel by mixing it with a carrier signal and filtering to limit the bandwidth of the transmitted energy, such that the energy radiated out of band is small enough not to interfere with communications in adjacent channels. Whilst in theory it is possible to provide any level of filtering in generating the signal to be transmitted, and to filter the received signal to remove energy from adjacent channels, there is a significant cost and power penalty in achieving high levels of signal rejection. In addition, whilst it is theoretically possible to generate a signal in the digital domain which is extremely well constrained in the frequency domain, in practice the signal is coupled to the medium through analogue circuitry. The inevitable non-linearity of real-world analogue components generally results in distortion of the signal, causing some energy to be radiated out of band.
In order to receive such a modulated signal it is necessary to down convert the signal to baseband, by mixing it with a replica of the carrier frequency and filtering the mixer output to extract the desired information-carrying component. This mixing can be done in the digital or analogue domain; however, digital mixing requires sampling the data at much higher rate—increasing the power and therefore cost of the digital components. Consequently, it is common to perform the down-conversion in the analogue domain, followed by filtering the output of the mixer to limit the bandwidth and then sampling into the digital domain using an Analogue-to-Digital Converter (ADC).
FIG. 1 illustrates a typical channelization of a full duplex communication system where the transmit (Tx) and receive (Rx) channels are separated in frequency by a distance significantly greater than the channel bandwidth. The plot shows the local transmit spectrum, normalized to 0 dB at the transmitter centre frequency. This is the typical signal at the output of the Tx filter. FIG. 2 shows a similar channelization of a full duplex communications system where the channel separation is significantly smaller in relation to the channel bandwidth. This plot shows the Rx spectrum with receive path attenuation of 60 dB, typical of the received signal at the media interface. In this case, the Tx signal remains adjacent to the Rx signal after down-conversion and therefore it needs a much sharper filter roll-off in order to prevent the Tx energy being present in the Rx signal. This implies that a significantly more complex and expensive solution is required.
In addition to this, there is a small amount of spreading which occurs due to non-linearities in the mixer. This results in a small amount of energy due to the near-end transmitter appearing at the output of the mixer in the receive-chain, which resides in the frequency band of interest. Although this may be attenuated compared to the total transmitted energy, it can still be very significant when compared to the energy received from the far-end transmitter, after accounting for signal transmission losses. Accordingly, this residual component of the (near-end) transmitted energy can limit the achievable sensitivity.
Since the filter roll-off in the analogue domain is often significantly limited by cost constraints, it is desirable to keep a large channel separation between the Tx and Rx channels compared with the channel bandwidth. Use of a large channel separation between the transmit and receive channels means that the near-end transmitter signal is more attenuated out-of-band by the filters in the receive-chain, prior to sampling to digitise the signal.
The sampling of the signal results in all energy in the signal being captured. Any energy which is present outside of the sampling bandwidth is aliased into the sampled signal. Once this has happened, the energy is within the bandwidth of the signal of interest and cannot be removed by a simple filtering process. The solution to this problem in a typical wireless communications system is to use a channel separation between the Tx and Rx channels which is much greater than the bandwidth of the signal being carried by the channel. In this way, after down-conversion of the receive channel, any energy from the transmit channel has a large frequency offset from the desired energy of the received channel and can be effectively removed using simple low-pass or band-pass filters prior to sampling. However, increasing the channel separation inherently means that spectral bandwidth is used less efficiently.
Due to spectrum availability and licensing it is not always possible to allocate channels for transmission and reception having the desired wide separation and it is necessary for the two channels to be adjacent to each other within a very narrow spectrum window. One example of this is the use of the CENELEC band B for power-line communications, where the spectrum from 95 to 125 KHz is available for use without access restrictions, subject only to regulations regarding maximum transmitted energy, and spurious emissions outside of this spectrum.